Electron discharge device



NW 12, W46. 1.. F. BROADWAY ET AL 2,419,853

ELECTRON DIS CHARGE DEVICE .Filed Dec. 29, 1942 TTO/TMEY Patented Nov. 12, 1946 2,410,863 ELECTRON DISCHARGE DEVICE Leonard Francis Broadway, Ickenham, and Cabot Seaton Bull, Hillingdon, England, assignors to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application December 29, 1942, Serial No. 470,506 In Great Britain March 5, 1940 2 Claims. 1

This invention relates to electron discharge devices of the velocity modulation type. Devices of this type find special application as ultrahigh frequency oscillators, amplifiers and detectors.

The usefulness of devices of this type increases with the magnitude of the beam current, and the magnitude of the beam current will, depend- .ing on the focussing action employed, determine the area of the transverse section of the beam. In order to increase the beam current a larger cross-section beam must be employed and if, as is usually the case, a beam of circular section is used it is found that the efficiency-of the tube is seriously diminished as the circular sectioned beam is enlarged since large diameter apertures in the tube electrodes are required which cause an unwanted radiation of energy together with an unwanted dissemination of electrostatic field which latter gives rise to increased transit time of the electrons necessitating the use of abnormally high voltages if the transit time is to be reduced to the required value. This difliculty is not overcome by providing grids in the apertures since at ultra-high frequencies the lengths of the wires of the grids present a high impedance. It is desirable in order to increase the efii'ciency of tubes of the type referred to, to maintain the ratio of the capacity between the electrodes or between said edges to the cross-sectional area of the beam as small as possible. It

' is found that, for equal values of the beam current, the above-mentioned difliculties can be reduced or overcome or the required conditions can be more conveniently satisfied by using a beam in the form of a ribbon.

According to one feature of the'invention there is provided an electron discharge device of the velocity modulation type provided with, means for generating an electron beam in the form of a ribbon.

shapedto conform generally to the transverse section-of said ribbon beam.

- The term ribbon is intended in this specification to include any beam the transverse section of which has one dimension substantially greater than the other. The ribbon may be an elongated rectangle, or may be elliptical, the major axis of which is long compared with the minor axis. The electron beam need not be in the form of a ribbon throughout its length providing it is of ribbon shape in the vicinity of said electrodes or resonator orresonators. The term longer sides of the beam means the longer sides of the beam whenviewed in transverse section.

The electrodes of the input or output system may be of any suitable construction. For example, the electrodes of the input or output system may consist of three rods adjacent one of the longer sides of the beam or three pairs of rods embracing both the longer sides of the beam or they may consist of three fiat strips disposed along one of the longer sides of the beam, or three pairs of fiat strips embracing both of the longer sides of the beam, said strips having appreciable extension in the direction of travel of the beam. Any other group of three electrodes may be employed which are capable of disposition near the beam without substantially obstructing the passage thereof. The first and third electrode of each group may have extensions, such as of solid or perforated sheet metal or metal gauze, extending to the wall of the envelope and preferably at right angles to the pathv or the beam. The longitudinal spacing of the three electrodes of each group is so related to the steady operating potentials that in the case where these electrodes have negligible extension in the direction of beam travel the time of flight of the electrons between the first and second electrodes, and between the second and third electrodes, is equal to half the time period of the high frequency potential, while in the case where these electrodes have appreciable extension in the direction of beam travel, the time of flight from the gap between first and second electrodes to the gap between second and third electrodes is equal to half the time period of the high-frequency potential. In some cases it may be desirable to employ a longitudinal magnetic field.

It may in some cases be desirable to interconnect the rods of the pairs of electrodes with transverse grid wires and since the rods can be arranged relatively close together due to the ribbon shaped beam employed the lengths of the wires will not present the difficulties hereinbefore referred to.

Where either the input or output system, or

3 generally comprise a hollow body surrounding the beam and instead of having circular apertures through which the beam passes, as in known forms of hollow resonators, the apertures are shaped to conform to the transverse section of the beam. If the beam is of elongated rectangular shape in transverse section, then the shape of the aperture in the resonator through which the beam passes will also generally be of an elongated rectangular shape, the resonator being suitably shaped accordingly.

In order that the invention may be more clearly understood and readily carried into effect it will now be more fully described with reference to the accompanying drawing, in which:

Figure 1 shows diagrammatically a plan View of the electrodes of a tube according to one form of the invention,

Figure 2 is a side elevation of a complete tube according to the invention,

Figure 3 is a transverse section of the tube shown in Figure 2, but with a modified con struction of electrode,

Figure 4 illustrates diagrammatically in crosssection a tube employing a hollow resonator constructed in accordance with the invention, and

Figure 5 illustrates a front elevation of the hollow resonator shown in Figure 4.

Referring now to Figure 1, l represents a thermionic cathode having a plane rectangular emitting surface with its lesser dimension parallel to the plane of the drawing and its greater dimension perpendicular thereto the cathode serving to emit a beam in the form of a ribbon 2 represents a control electrode comprising the rods 3, 41 disposed symmetrically to the sides of the cathode at a short distance in front thereof and supporting a number of wires 5 parallel to the plane of the drawing. 6 is an accelerator electrode of the same construction as the control electrode 2, and disposed at a short distance in front thereof. The input and output systems in the example shown each comprise three electrodes consisting of pairs of strips 8, 9 and la, 8a, 9a respectively, spaced apart along the length of and disposed symmetrically with respect to the beam l0, and closely adjacent thereto. The electrodes lie along the whole length of both of the longer sides of the beam. For most efilcient the electrodes 8 and 8a respectively. This will have the effect of focussing electrons through the electrodes 8 and 8a thus reducing the D. C. current to these electrodes and maintaining a high value of D. C. impedance. The electrodes 9 and 9a may be at any suitable steady potential consistent with the consideration that the time of flight of the electrons from the gap between electrodes I and 8 or la and 8a to the gap between electrodes 8 and 9 or Ba and 9a is equal to a halfperiod of the high-frequency potential. 'Electrodes 7 and 9 and l'a and 9a are earthed as regards high-frequency and the fluctuating potentials are borne by electrodes 8 and 8a.

It is found advantageous to operate the con trol electrode 2 at a low positive potential and the accelerator 6 at a somewhat more positive potential, since the preliminary focussing of the electrons achieved in this manner tends to equalise the times of flight in the axial direction of all the electrons of the beam, and hence to produce more complete velocity modulation and higher output.

In Figure 2 the various electrodes have the same reference numerals as in Figure 1. [4 represents the envelope of the device and I5 a seal of a particularly suitable type for introducing the high-frequency conductors into the tube. The capacity per centimeter in -farads of the electrodes 8 to electrodes 1 and 9 together and of electrodes 8a to electrodes la and 9a is arranged to be the same as that of the transmission line or resonant circuit to be connected to them, and is given by the relation:

where Z0 is the impedance of the transmission line measured in ohms, and K is the dielectric constant of the medium in which the electrodes are situated measured in electrostatic units. If the electrodes '1, 8, 9 and la, 8a, 9a, consist of pairs of rods with transverse wires, the capacity is approximately equal to:

JWL 47K d Z0 neglecting edge eilects where W is the lateral separation of the rods and d is their longitudinal separation. The correct relations for other electrode constructions may be found either experimentally or by calculation.

Although in the examples described with reference to the drawing, the input and output systems are both inductively coupled with the electron stream, it is to be understood that the invention is not limited to such a construction. For example, in some cases the output from the tube may be taken from an anode on which the electron stream impinges.

Furthermore, the invention is not limited to the types of input and output systems shown in Figures l to 3 of the drawing since the invention finds a ready application to the type of velocity modulation device in which the input or output system, or both, comprises a hollow resonator through which the beam passes. In this type of velocity modulation tube the hollow resonator is tuned to a desired frequency and the potentials which impart the velocity modulation to the beam can be applied to one hollow resonator and the output energy abstracted from another resonator suitably spaced from the first resonator. In velocity modulation devices employed as oscillators one resonator may be suitably coupled to another resonator or a. single resonator may be employed and the velocity modulated beam may be reflected back into the single resonator. The in- 5 vention can be applied to any of these or other types of velocity modulation devices employing hollow resonators. Figure 4 of the drawing illustrates diagrammatically in cross-section with the envelope omitted, a device employing a single hollow resonator constructed in accordance with the invention. In this device the cathode l is of elongated form and may be associated with other electrodes so as to cause it to generate a ribbonshaped beam. The ribbon-shaped beam is arranged to pass through an aperture 20 in a hollow resonator 2| the aperture being shaped to conform generally to the transverse section of the ribbon beam employed, the edges of the aperture being disposed as close to the beam as possible. The beam after passing through the resonator is reflected by a reflecting electrode 22 back through the resonator. The resonator as shown in Figure 5 generally comprises a pair of straight cylindrical portions 25, 24 lying along the longer sides of the beam, the ends of said portions being interconnected by hollow semicircular portions 25, 25. The resonator is arranged generally in a plane at right angles to the axis of the beam and surrounds the beam, the resonator as will be observed being similar to known forms of resonator with the exception that the aperture is shaped to enable the ribbon beam to pass therethrough and the general configuration of the resonator shaped accordingly.

What we claim is:

1. An electron discharge device having a cathode for supplying a single beam of electrons along a beam path, said beam of electrons having an elongated rectangular transverse cross-section, a collector for receiving said beam of electrons, and electrode means positioned between the oathode and the collector and including a plurality of pairs of successively positioned ribbon-like elongated narrow electrode elements of conducting material, said conducting elements lying parallel to the longer axis of the beam path, the elements of each pair of electrodes being equal in width and spaced apart transversely of the beam path a distance equal to the width of said elements parallel to the direction of movement of the electrons along the beam path.

2. An electron discharge device having a cathode for supplying a single beam of electrons along a beam path, said beam of electrons having an elongated rectangular transverse cross-section, a collector for receiving said beam of electrons, and electrode means positioned between the cathode and the collector and including a plurality of pairs of successively positioned ribbon-like elongated narrow electrode elements of conducting material, said conducting elements lying parallel to the longer axis of the beam path, the elements of each pair of electrodes being of equal width and spaced apart transversely of the beam path a distance equal to the width of said elements parallel to the direction of movement of the electrons along the beam path, the distance between successive gaps'between adjacent pairs of elements along the beam path being equal to the distance that an electron travels during a half period of the high frequency at which said electron discharge device operates.

LEONARD FRANCIS BROADWAY. CABOT SEATON BULL. 

